The present invention relates to a variable assist power steering system, and more particularly to an arrangement to bypass fluid flow so as to have the degree of power assistance change with vehicle speed or some other variable related to the mode of operation of the vehicle.
A power assisted steering system can be characterized as operating under three driving conditions. Firstly during straight ahead driving at medium to high speeds, the power requirements on the steering system are extremely low and the degree of power assistance provided by the steering gear should correspondingly minimized to permit the feedback of road "feel" from the tires to the driver. Secondary during medium and high speed passing and cornering maneouvres, a progressive increase in the level of power assistance with driver input torque is desirable. Nevertheless moderate driver input torques should still be maintained in order that the driver can feel adequately the dynamic state of the vehicle. Thirdly, and lastly, during low speed or parking maneouvres, the power requirements on the steering system may be large and fidelity of the steering system in terms of transmitting road feel is of little importance. Under these circumstances it is generally desirable to offer large degrees of power assistance, thereby minimizing the input torque required to be furnished by the driver.
The demands for optimum valve characteristics during the above three driving conditions conflict. Attempts have been made in the past to avoid the conflicting demands of the first and third driving conditions, namely the need to achieve a low level assistance for high to medium speed on-center driving while having high levels of assistance for low speed and parking maneouvres, by exploiting the fact that, for most valves, the degree of assistance varies with the flow of oil. For example, in a known system as disclosed in JP No. 56-38430 B2, a bypass path with a solenoid operated variable flow orifice valve is connected between both ends of the power cylinder and the degree of opening of the variable flow orifice valve is controlled in response to vehicle speed to cause bypass flow to increase as vehicle speed increases. A potentional problem encountered in this system is that it is difficult for a variable flow orifice valve to exibit the same pressure drop characteristics for fluid flow passing therethrough in one direction as those for fluid flow passing therethrough in the opposite direction.
An object of the present invention is to improve a variable assist power steering system such that the fluid of a bypass flow is directed to pass through an externally controlled variable flow orifice in one direction only.